A class of brownmillerite-derived materials have been found to be mixed ionic and electronic conductors (MIEC). See: PCT/US96/14841, filed Sep. 13, 1996 and U.S. patent application Ser. No. 09/314,708, filed May 19, 1999. Gas-impermeable membranes made from MIECs behave as short-circuited electrochemical cells, in which oxygen anions and electrons conduct in opposite directions through the membrane. The oxygen anion conductivity of these materials make them useful for the efficient generation of pure oxygen which can be employed in catalytic oxidation reactions. The electronic conductivity of these materials provides for spontaneous gas phase separation and any subsequent oxidation without the addition of external electronic circuitry. MIEC membranes have found application in catalytic membrane reactors (CMRs) for a variety of processes where oxygen reacts with inorganic or organic species, such as hydrocarbons, including but not limited to the partial oxidation of methane and other hydrocarbons to synthesis gas, the oxidative coupling of aliphatic and aromatic hydrocarbons, and the gasification of coal. CMR processes can also include decomposition of organic environmental pollutants such as PCBs. These membranes can simply be used for the separation of oxygen from oxygen-containing gas (e.g., air) and the production of pure oxygen.
MIEC brownmillerite-derived ceramics are of general composition: EQU A.sub.x A'.sub.2-x-y A".sub.y B.sub.z B'.sub.2-(z+z") B".sub.z" O.sub.5+.delta.
where A is an element from the fblock lanthanide elements; A' is an element selected from the Group 2 elements, A" is an element from the fblock lanthanide or Group 2 elements; B is an element selected from Al, Ga, In or mixtures thereof; B' and B" are different elements and are selected independently from the group of elements Mg, and the d-block transition elements, including Zn, Cd, or Hg; 0&lt;x&lt;2, 0.ltoreq.y&lt;2, 0&lt;z&lt;2, and 0.ltoreq.z"&lt;2, where x+y.ltoreq.2, z+z".ltoreq.2 and .delta. varies to maintain electroneutrality. A" and B" may or may not be present.
The lanthanide metals include the fblock lanthanide metals: La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Yttrium has properties similar to the fblock lanthanide metals and is also included herein in the definition of lanthanide metals. Ln is preferably La or mixtures of La with other lanthanides. The more preferred B elements are Ga and Al. The d block transition elements include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn. Preferred M are Fe, Fe mixed with Co, with Fe being more preferred as M.
Ceramic materials typically have low coefficients of expansion, and are inherently brittle. Metals in general have higher coefficients of expansion and are more ductile. The difficulties in implementing ceramic materials in existing hardware designs often result from the differing properties of the materials. The expansion of many mixed conductors is quite high. This arises from the combination of both thermal expansion and chemical expansion. The thermal expansion common to most materials arises from the increased amplitude of atomic vibrations. The increased amplitude of vibration leads to increased atomic separation, corresponding to a lattice expansion. Certain materials also have a significant, nonzero, chemical expansion. This arises from a change in composition as a function of temperature, without a change in crystal structure. In the case of these metal oxides, the chemical expansion is a result of the change in oxygen concentration as a function of temperature. Metal oxides heated in air are slightly reduced. The high ionic conductivity of these materials suggests high ion mobility and high vacancy concentration. The membrane material has the ability to accommodate a high number of vacancies. The vacancy concentration changes as a function of temperature and oxygen partial pressure. Large expansion can cause excessive stresses in reactor design.
In operation, the ceramic membranes of CMRs are exposed to extreme operating conditions. The opposite surfaces of the membranes are simultaneously exposed to a highly oxidizing atmosphere and a highly reducing atmosphere, respectively, at high temperatures. Also, the chemical and thermal expansion of the ceramic membranes in CMRs can lead to excessive stresses on the reactor. Thus, there is a need in the art for ceramic membrane materials that retain excellent ionic and electronic conductivities and which exhibit chemical stability with respect to decomposition and, in addition, exhibit low expansion properties.